1. Field of the Invention
The present invention relates to artificial hip-joint sockets suited for cement-free implantation into the acetabulum.
2. Description of Related Art
Hip-joint sockets adapted for cement-free implantation are known in the art. Such hip-joint sockets have a wear-resistant inner shell comprising a cylindrical salient housing a longitudinally mounted comb, and n intermediate polyethylene shell being fitted with a corresponding recess having an additional elongated slot at its base. The cylindrical salient and the corresponding recess guide the inner and outer shells. The longitudinal comb fitting into the elongated slot secures the two shells against relative rotation. This pre-assembled system may be detachably connected to an outer metal shell during surgery. This connection is implemented using a deforming polyethylene detent device. The titanium or titanium-alloy outer shell is connected to the bone without resort to cementing.
This known hip-joint socket incurs the drawback of a substantial height of the wear-resistant inner shell caused by the superposition of the guide elements and the rotationally locking elements. Another drawback is that the longitudinal comb of the inner shell applies, on account of its geometry, concentrated stresses at the inner side of the elongated slot of the intermediate shell. As a consequence ceramic materials, which disadvantageously react to such stresses, cannot be used.
The present invention is directed toward an artificial hip-joint socket implementing optimal spreading of the applied forces at minimal design height.
The design of the invention offers the advantage that the prismatic elements at the inner and intermediate shells simultaneously guide the shells and also secure them against rotation and canting. The inventive design, as a result, has a reduced height. This reduced height, on the one hand, permits smaller sockets and lesser wall thickness of the intermediate shell, while, on the other hand, even very small sockets allow congruence of the radius insertion point relating to inner slide surface and outer geometry. This feature amounts to kinematically optimal hip-joint reconstruction.
Compared with the state of the art, the present invention offers improved, omnidirectionally more uniformly distributed canting and rotational forces between the inner and the intermediate shells. In other words, torques exerted by the joint""s ball on the inner shell are optimally transferred by means of the intermediate and the outer shells to the hip bone.
In a preferred embodiment, the salient of the inner shell and the corresponding recess in the intermediate shell each substantially represent a regular tetragonal prism. In this manner, the applied forces are spread or distributed optimally. The length of the regular sides of the tetragonal prism may be in the range of 10 to 18 mm, preferably 13 to 15 mm.
The elevation of the prismatic salient and that of the prismatic recess is in the range of 2 to 6 mm, preferably 3.5 to 4.5 mm. The facets implemented by rounding or cutting further reduce the wall thickness at the intermediate shell and this feature enables system integration into small outer shells and, hence, also into small acetabuli.
The overall height of the inner shell, inclusive the salient, shall be at most 65%, preferably at most 60% of the maximum diameter of the inner shell (3).
Preferably the wear-resistant inner shell is composed of a CoCr alloy or of a ceramic such as Al2O3.